1. Field of the Invention
The present invention relates to a photodetector circuit that determines illuminance by taking a thermoelectric current into account.
2. Description of the Related Art
Liquid crystal display devices, which are light in weight, thin in shape, and low in power consumption, are frequently used as the display devices for information devices and for mobile terminal devices. To achieve a brighter display screen, many liquid crystal display devices are provided with backlights for emitting light from the back side of the liquid crystal panel. The necessary illuminance of the backlight for sufficient visibility differs depending on the brightness of a place where the liquid crystal display device is used. To detect the difference, some display devices have photosensors to determine the illuminance of the outside light to adjust the illuminance of the backlight. A known example of such display devices is disclosed in Japanese Patent Application Publication No. 2007-114315.
Integration of the photosensor into the liquid crystal panel is advantageous for the downsizing of a liquid crystal display device. However, the photosensor that is formed integrally with the liquid crystal panel by the low-temperature poly-silicon technique has its own drawbacks. For example, the photosensor thus formed is likely to be affected by the thermoelectric current, which in turn makes it difficult to accurately determine the outside-light illuminance. To address this problem, the display device disclosed in the above-mentioned document is provided not only with a first photosensor to receive the outside light and to determine the illuminance thereof but also a second photosensor to determine the thermoelectric current while being shielded from the outside light. Calculating the difference between the two values thus determined gives a more accurate illuminance of the outside light.
However, in some cases, the first photosensor and the second photosensor have different levels of thermoelectric currents due to a variation in characteristics between the two photosensor elements and a difference in the temperature distribution between the positions where the two photosensors are respectively arranged. This causes a problem that a stable photodetector circuit cannot be provided.